<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>mydemo</title>
	<style type="text/css">
		div {text-align:center;}
	</style>
	<script type="text/javascript">
		window.onload = function() {
			var oNum = document.getElementById("num");
			var i = 0;
			setInterval(sum,1000);
			sum();
			function sum() {
				oNum.innerHTML = ++i;
			}
			
		}
	</script>
</head>
<body>
	<div><strong id="num"></strong></div>
</body>
</html>